RVGL source code is private. Few people are working on it, that's how you get new builds regularly.

If you're asking where is RVGL coming from, well, it's a rewrite of v1.2 community patch, which was based on the leaked xbox version source code of the game.
Development went quite a long way (over 8 years), so first public builds of 1.2 were released somewhere in 2010, with the last one being 15.0420, before RVGL completely took over.